The present invention relates to nuclear power reactors, and more particularly to a method and apparatus for detecting thermal hydraulic oscillations in the core of a boiling water reactor.
By the nature of its design, a boiling water reactor (BWR) operates at or near the nucleate boiling point of the coolant which flows through the reactor core. During certain startup conditions, before the main coolant pumps are started, but while operating in the power range under natural circulation flow, voids (steam) can appear within the active fuel region of the core. These local voids are less dense than the surrounding coolant and this results in a decrease in the local moderation of neutrons. The decrease in moderation and thermalization of neutrons, results in a decrease in the nuclear fission rate in the nearby fuel. Such localized depression of the thermal neutron flux can, under certain conditions, cause a power unbalance and thermal-hydraulic instability in the reactor. These can rapidly progress to large thermal hydraulic oscillations.
Boiling water nuclear reactors typically contain a variety of instrumentation, including a plurality of local power range monitors (LPRM) distributed throughout the core. The LPRM is typically responsive to thermal-neutron flux and thus responds to localized flux depressions resulting from localized voiding. One known approach for detecting thermal hydraulic oscillations, uses the LPRMs and monitors the peak-to-peak level between selected quadrant symmetric LPRMs. When the peak-to-peak flux levels exceed a predetermined limit, an alarm or automatic corrective action is initiated.
This prior art technique relies on stochastic signal noise analysis. Because noise analysis is computationally time-consuming, this conventional technique provides a relatively late indication of the occurrence of a problem. In other words, the oscillation is well underway before the detection system of the prior art generates an alarm or initiates corrective action.